Withdrawing Pub/Sub-triggered Cloud Function drops pending messages

Google Cloud Functions can be triggered by messages coming into a Pub/Sub topic subscription. However, such subscription is automatically created when the function is deployed, and then is automatically destroyed when the function is withdrawn (deleted), thus losing all the messages pending in it.

This affects our `kcidb_spool_notifications` Cloud Function. We lose all unprocessed object updates when withdrawing it. This is especially a problem when the subscription is congested, e.g. due to the function timing out a lot, as is the case in #360.

Start by tracing how kcidb_spool_notifications is deployed and withdrawn, and review the related timeout context in #360. The work is done when withdrawing the function no longer loses pending object updates from its subscription.

Written by the indexing model from the issue text.